One point that creates confusion, here, is that Illinois (along with Hawaii) allows civil unions for heterosexual couples. The law, here, changes frequently, but I think all other states that allow civil unions limit them to same-sex couples. So, in those other states one has a more clear presentation of the differences between "marriage" and civil unions.
There are three big differences:
1) Federal benefits: The Defense of Marriage Act (1996 -- thanks, Bill Clinton) defines at the federal level that a marriage is between "one man and one woman." That means that, for things like federal taxes and Social Security benefits a civil union is meaningless. "Civilly unioned couples" (that's what we call them) -- whether same-sex or heterosexual -- don't have the same benefits that married couples do. Taxes and Social Security are the tip of the iceberg, though. The rights conferred by the word "marriage" spread all over federal law into things like immigration, bankruptcy, etc. The only solution lies in a repeal of DOMA or a Federal Civil Union law.
2) Portability: A heterosexual couple that marries anywhere in the world is married in any of the 50 states. A same-sex couple that marries in a say, New York, however, isn't necessarily married everywhere else. In fact, they're not married at all in most states. It's not necessarily true that they can revert to the "civil union" rights of the new state, either. So, they marry in New York, get the state benefits but lack the federal benefits, and then move to, say, Idaho, where they lose even the state benefits of being married or civilly unioned.
3) Culture. A guy named Paul Cates (of the ACLU) put it this way: "“You’re not a little kid dreaming about your civil union day. It’s your wedding day. When you want to commit to a partner, you’re not really thinking about the [legal] protections . . . . It’s the significance and what it means to be married and hold yourself out as married.” It's the least concrete difference . . . and probably the most significant.
This is indeed a complex issue as there are many more differences between civil marrianges and civil unions. I suggest you research this issue much more throughly before making any type of decisions relevant thereto. In Illinois gay couples can are only legally able to enter into Civil Unions. Same sex couples can enter enter either Civil unions or civil marriages. If you are married in another state that allows legal gay marriage you will be recognized in Illinois as only a Civil Union and NOT a Civil Marriage. Basically Civil Unios provide none of the protections or responsibilities federal law provides married couples. Some of the glaring differences are not being abl to file joint tax returns. Not being able to receive survivor social security bebefits. No federal veteran's spousal benefits. Most other states will NOT recognize an Illinois Civil Union. You have no immigration right's associated with marriage. Civil unions do allow the partners to make health care decisions regarding each other and also enter the hospital room of their partner. As to the religious differences I cannot comment. You have to talk to your pastor, rabbi, prienst etc. to talk about the religious significance. I am sure you have heard that Illinois seems to be very close to passing a neew law allowing legal marriages between same sex partners. If that occurs I do not know if the civil union law wil stay on the books. I suspect that it will probably be done away with for the future.
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